1. Technical Field
This invention relates to the field of electromagnetic radiation source devices and apparatuses for testing accelerated weathering of samples under the effect of light, and other atmospheric conditions such as temperature, humidity or water precipitation. In particular, the tests relate to the resistance of samples made of polymer materials that are essentially sensitive to ultraviolet radiation.
2. State of Prior Art
Natural weathering of materials under solar light is simulated in a known manner by exposing samples to an electromagnetic radiation source with a spectral distribution that must be carefully chosen to achieve accelerated weathering that can be correlated to weathering observed under natural conditions.
Various devices and apparatuses are known for controlling exposure to laboratory light sources. The nature of light radiation sources has an overriding influence on test conditions and fully determines operation of the devices.
Thus standard ISO 4892 that governs ‘Exposure methods to laboratory light sources’ for “Plastics”, makes a distinction between three radiation source categories, and apart from the General Guide in Part 1 (ISO 4892-1), contains three appendices dealing with the following subjects:                Part 2: Xenon arc sources (ISO 4892-2)        Part 3: U.V. fluorescent lamps (ISO 4892-3)        Part 4: Open flame carbon arc lamps (ISO 4892-4).        
In the category of carbon arc lamps, historically the oldest category, the light source is formed of several pairs of carbon electrodes, typically 2 to 4 pairs of electrodes, composed of a mix of rare earth metallic salts with surface metallisation, the opposite electrodes being arranged axially one along the prolongation of the other with a system for keeping their spacing constant as they are consumed.
The electric arc between the carbon electrodes generates radiation that contains ultraviolet rays, visible light and infrared light, while releasing a very large quantity of heat.
Consequently, the samples cannot be directly exposed to the open flame in carbon arc lamps and light must be filtered, different filters and protection elements being recommended.
ISO standard 4892-4 recommends glass filters resistant to heat, which is suitable to simulate exposure to sunlight through thick window glass, Pyrex, Corex, etc.; these glass filters must have a composition and thickness giving a specific transmission spectrum.
A series of documents related to devices for accelerated tests on stability to light and climatic agents is known in this category of devices and apparatuses equipped with carbon arc lamps, produced by the Japanese SUGA Test Instrument Co. Company.
Document U.S. Pat. No. 4,760,748 published by SUGA in 1988 and the previous documents series U.S. Pat. Nos. 4,704,903 and 3,983,742 all describe devices consisting of a large solid cylindrical frame that rotates around a light source composed of a vertical tubular funnel containing pairs of electrodes arranged longitudinally. The polymer sample test pieces are arranged in outlets formed in solid walls of this cylindrical frame.
Windows are perforated in the funnel (or stove pipe) in which the filters are located.
The funnel or the cylindrical casing that supports the glass filter elements is thus inserted between the carbon electrode arc burner and the samples that are fitted in openings in the tubular sample-holding frame to prevent the strong infrared radiation and the high release of hot air from the carbon electrodes flame from reaching the surface of the samples.
The sample-holding frame rotates around the central axis of the carbon arc burner casing, which is vertical or slightly inclined according to the recommendations in ISO 4892.4 (point 4.2). The sample-holding frame has a large overall size, about a meter and typically 96 centimeters.
Due to the large release of heat, carbon arc lamp devices have a complex double cooling system, firstly to cool the burner, and secondly to cool the samples.
Devices comprising a fixed header pipe of low-pressure arc tubes that supply ultraviolet radiation are known. This ramp of parallel tubes is arranged facing the samples to be tested.
The disadvantage of the radiation from low-pressure arc tubes is that emissivity is low and the ultraviolet spectrum is very different from the solar spectrum, which has a negative effect on the tests.
Another disadvantage is that samples cannot be exposed uniformly in these devices, such that the reproducibility of test results is not good.
Devices using Xenon arc lamps are known, which have the advantage of having a spectrum very similar to the solar spectrum when they are suitably filtered.
According to the state of recent developments in the art, it is now possible to adopt filtered medium pressure mercury vapour lamps with a spectrum very rich in ultraviolet radiation and with a good equilibrium between long U.V.A. type ultraviolets and short U.V.B. type ultraviolets, in an equivalent manner, to achieve good reproduction of weathering due to sunlight.
Weathering test apparatuses are known in which samples are mounted on a vertical cylindrical sample holder rotating around tubular Xenon arc lamps arranged vertically.
Rotation of samples around Xenon lamps provides a means of achieving uniform exposure to radiation.
However, these lamps release a great deal of heat and open circuit or closed circuit air circulation is necessary to keep the samples at a controlled temperature.
These apparatuses have the disadvantage that they expose the samples to variable temperatures.
The ventilation circuit comprises an air intake zone above the cylindrical sample-holder that creates a vertical air stream like in a funnel. The air stream warms up at it moves vertically along the lamps and the samples, so that the samples at the top of the sample holder are necessarily warmer than the samples at the bottom. Therefore, identical samples at different heights are subjected to different temperatures, and this also affects the reproducibility of the tests. In practice, some users are obliged to interrupt the tests to invert the samples being tested, and swapping the positions of the samples between the top and bottom of the sample holder.
Devices are also known comprising a parallelepiped shaped test chamber with mercury lamps arranged vertically in the four corners of the chamber around a small rotating sample-holding cylinder arranged in the centre of the chamber. The lamps and the outer face of the samples are cooled by air circulation ventilated with air inlets open in the sidewalls of the chamber and an air opening covering the entire sample-holding cylinder at the top of the chamber.
One disadvantage of this device is that it is excessively large compared with the small area of the samples being tested.
This device also has the disadvantage that it has large light energy losses, since more than 75% of the light does not travel directly towards the sample holder.
Consequently, the efficiency of this device per unit area of samples being tested is low, when the depreciation of the equipment plus energy expenses are included, making the tests expensive.
There are also apparatuses comprising a rectangular test chamber in which a cylindrical sample-holding cage is mounted, free to rotate about a solid drive shaft. A laminar air stream travelling vertically between openings formed along the axis of the generating lines of the cylinder at the floor and at the top of the chamber passes along the samples, in order to isolate the samples from the hot air stream from the lamp and to homogenize the test temperatures.
However despite the laminar air stream, this instrument still has the disadvantage that it exposes the samples to different temperatures, the laminar air stream becoming warmed further as it comes into contact with samples exposed to bright light radiation. In practice, samples arranged at the top of the cage are thus exposed to a temperature several degrees higher than the samples at the bottom of the cage.
Patent EP-0 320 209 thus describes an atmospheric test cabinet comprising a sample-holding rack or basket rotating around a vertically arranged xenon light tube. The samples are cooled by a laminar air stream that passes vertically along the internal walls of the samples arranged along the straight edges of the rack. The air stream has a limited flow rate for reasons of economy.
This vertical low flow rate laminar air stream test cabinet has the disadvantage that it does not provide efficient cooling of the samples and it induces temperature differences between samples at the bottom and samples at the top of the rack.
Patent U.S. Pat. No. 4,760,748 describes another accelerated weathering test device also comprising a cylindrical sample-holding frame free to rotate about a vertical axis and light tubes, with an ascending laminar air stream cooling the inner face of the samples. The cylindrical frame comprises solid walls through which two rows of openings are perforated, firstly to place the samples and secondly to form intake orifices for a secondary air stream.
The device is cooled by providing it with a double flow air circulation system, with an ascending main air column rising in the sample-holding cylinder, passing along the inner face of the samples and a peripheral secondary air stream that strikes the outer face of the samples. The makeup secondary air stream originates from a cooled air source. The secondary air stream is created by a suction effect provoked at the orifices of the cylinder by ascension of the main air column. The size of the orifices can be varied to modulate the peripheral secondary air stream as a function of the ascending main air stream.
The disadvantage of this device is the complexity and the particularly large size of its double flow air circulation system.
Another disadvantage with this device is that its testing space and the number of samples that it can hold are very limited, considering the large size of the cabinet including its equipment for evacuation of air near the top, the blower and the rotation drive near the bottom, and the peripheral air circulation system and the lateral cooling air source.
This double air stream cooling system has another disadvantage, which is that it induces significant temperature differences between the irradiated inner face and the cooled outer face of the samples.
German document DE-A-32 43 722 describes a very different device for tests on resistance to light and the weather, including a large ventilated chamber in which a horizontal assembly is arranged comprising a ring of light tubes inserted between two concentric horizontal air ducts and a sample-holding drum mounted free to rotate about the horizontal ducts. The two ducts are connected to a fan and communicate with each other and with the outside of the chamber, such that external air is blown into the first duct and is then returned and enters the space between the first and the second duct to pass along the light tubes, cooling them on its way back towards the outside.
The samples arranged on the outer drum are cooled separately by another vertical air stream that circulates in a circuit provided with another fan and a cooling exchanger, and possibly heating means.
Therefore, this device comprises two cooling air circulation systems, one cooling the samples and the other cooling the light tubes.
In this device, the light tubes and a sprinkler system are arranged in the ducts that separate them from the samples, but this has the disadvantage that it hinders light exposure of the samples and reduces the efficiency of the system.
Another disadvantage of this device is that the vertical air stream that passes through the sample-holding drum does not cool the samples efficiently.
The purpose of this invention is to make a device for exposure to radiation to test weathering of samples to submit the samples to a uniform temperature and irradiation, without the above mentioned disadvantages.
In particular, the purpose of the invention is to enable efficient ventilation of the samples during tests carried out with the device.
Another purpose of the invention is to obtain a device with a small number of lamps and a compact size.
Another parallel purpose of the invention is to obtain a device with a simple design and a high light efficiency as a function of the surface area of exposed samples so as to reduce the cost of the tests.
Finally, another purpose of the invention is to facilitate manipulation of samples prior to the weathering tests.